1. Field of the Invention
The present invention relates to an ultrasonic diagnosis apparatus for radiating ultrasonic waves in a cross section of a subject, detecting the intensity of reflected echo, and brightness-modulating the intensity of the reflected echo, thereby obtaining a tomogram of the subject, and also for detecting a frequency-deviation (Doppler shift) of the reflected echo and detecting the direction and flow velocity of a blood, flow in the subject, thereby displaying the blood flow two-dimensionally by coloring the blood flow portion in the tomogram in accordance with the detected flow velocity and direction.
2. Description of the Related Art known as a color Doppler flow mapping (CDF) apparatus, and in particular as a BDF apparatus, since it relates to a blood flow imaging in a B-mode display. A blood flow, which approaches an ultrasonic probe, is colored in red, a blood flow, which moves away from the probe, is colored in blue, and a turbulent flow is colored in green. The velocity of the blood flow is represented by brightness.
The BDF apparatus will now be described in brief. An ultrasonic Doppler method utilizes an ultrasonic Doppler shift wherein, when ultrasonic waves are reflected by a moving body, the frequency of the reflected waves shifts from a transmission frequency in proportion to the velocity of the object. Specifically, ultrasonic waves are radiated to a subject and the radiation direction is scanned in order to obtain a tomogram. In this case, ultrasonic pulses are transmitted repeatedly in respective directions in which ultrasonic waves are radiated, and a Doppler shift frequency is detected based on the phase variation of the reflected echo. Thus, the data representing the movement of the moving body at a depth, at which the echo is reflected, is acquired. According to the ultrasonic Doppler method, it is possible to know the direction of the blood flow at a location in the subject and the condition of the blood flow (e.g. turbulent flow or regular flow).
In order to obtain blood flow data from an ultrasonic reflected echo signal, an ultrasonic probe is driven to repeatedly radiate ultrasonic waves in a raster direction for a number of times, and the received signal is detected by an orthogonal phase detecting circuit, thereby obtaining a Doppler shift signal on the basis of blood cells. Since a color Doppler image is obtained in real time, the Doppler shift signal is frequency-analyzed by a frequency analyzing circuit to find an average value of Doppler shift, a variance value of Doppler shift, an average power of Doppler shift, etc. A blood flow velocity color flow mapping image is obtained by an auto-correlation circuit, etc. built in the frequency analyzing circuit, and two-dimensional blood data is displayed on a TV monitor. Recently, this apparatus has been used to diagnose not only the heart but also the part in which blood flow velocity is low, for example, blood vessel in the abdomen or peripheral blood vessel.
The conventional BDF apparatus, however, has the following problem. In order to obtain the two-dimensional blood flow data, as stated above, it is necessary to transmit and receive ultrasonic waves in one ultrasonic raster direction for a number of times. In addition, in order to observe the blood flow with a low velocity, it is necessary to continue observation for a long time in one radiation direction. Consequently, a long time is needed to acquire one BDF image. For example, when the blood flow in the abdomen is observed, a TV monitor is normally capable of displaying only four to ten frame images in every second.
On the other hand, about 30 frame images per second are necessary in order to enable a person to observe images smoothly and clearly, which are displayed on a monitor employing a standard TV system such as NTSC. The above-mentioned four to ten frame images per second result in discontinuity and unclearness in images.